1. Field of the Invention
The present invention relates to an apparatus for detecting a displacement of an electromagnetic actuator driven by a pulse width modulation signal. The electromagnetic actuator is used for generating a reaction force with respect to manipulation by a user and for allowing operating elements or members, valves and the like to be automatically operated, for instance.
2. Discussion of Related Art
In a keyboard apparatus of an electronic keyboard musical instrument, a drive mechanism is provided for each key to execute a control for increasing and decreasing a key touch feeling felt by a performer or player in a performance operation, i.e., a force sense control, or for allowing keys to be automatically operated without manipulation of the player to realize automatic performance, as disclosed in Patent Literature 1 below. The keyboard apparatus utilizes a two-way driving electromagnetic actuator. More specifically, in the electromagnetic actuator, a forward solenoid (forward coil) and a yoke configured to pivot a key in a key depression direction are disposed in series with a backward solenoid (backward coil) and a yoke configured to pivot the key in a key release direction. Further, a plunger (movable core) is inserted through the centers of these drive coils such that a drive shaft of the plunger contacts the key.
In the meantime, there is known a differential transformer as disclosed in Patent Literature 2 below for detecting a displacement position of a movable core by detecting a voltage induced by magnetic coupling.
Further, an electromagnetic actuator is used in a valve drive device of an internal combustion engine as disclosed in Patent Literatures 3 and 4 below.